In various electrophotographic image forming apparatuses, a developing roller and a charging blade (layer regulating blade) kept in press contact with an outer peripheral surface of the developing roller are used for developing an electrostatic latent image formed by exposing a surface of an electrically charged photoreceptor drum into a toner image.
That is, when the developing roller is rotated in press contact with the charging blade, toner is electrically charged. The electrically charged toner adheres to the outer peripheral surface of the developing roller and, at the same time, the amount of the toner adhering to the outer peripheral surface is regulated by the charging blade. Thus, a toner layer having a generally even thickness is formed on the generally entire outer peripheral surface of the developing roller.
In this state, the developing roller is further rotated to transport the toner layer to the vicinity of the surface of the photoreceptor drum. Then, the toner of the toner layer is selectively transferred onto the surface of the photoreceptor drum according to the electrostatic latent image formed on the surface of the photoreceptor drum. Thus, the electrostatic latent image on the surface of the photoreceptor drum is developed into the toner image.
The developing roller typically includes a roller body, and at least an outer peripheral surface of the roller body is formed of a semiconductive rubber (see Patent Literature 1).
Opposite ends of the outer peripheral surface of the roller body of the developing roller are respectively sealed with seal members to prevent toner adhering to the outer peripheral surface from leaking out from the ends. The seal members are made of, for example, a felt material, and fixed to a housing or the like of the image forming apparatus in sliding contact with the opposite ends of the outer peripheral surface of the roller body of the rotating developing roller.
In an image forming apparatus of a highly durable design requiring maintenance service after every time about 8000 images are formed, the toner is liable to leak from the opposite ends of the developing roller (that should have successfully been sealed with the seal members) before the first maintenance service after the start of the use of the apparatus. This occurs supposedly because opposite end portions of the outer peripheral surface of the roller body are worn due to the sliding contact with the seal members to create gaps between the seal members and the opposite ends of the outer peripheral surface.
Patent Literature 2 discloses that substantially the entire outer peripheral surface of the roller body is coated with a resin coating layer such as of a fluororesin or a silicone resin for stabilization of toner chargeability. The provision of the resin coating layer is expected to suppress the wear of the opposite end portions of the outer peripheral surface of the roller body which may otherwise occur due to the sliding contact with the seal members.
However, the resin coating layer is generally designed to have a friction coefficient μ of about 0.25 to about 0.5 in consideration of the balance of frictional force between the toner and the resin coating layer. Therefore, the resin coating layer has an insufficient wear resistance. Particularly, where the developing roller is used in the image forming apparatus of the highly durable design, it is impossible to prevent the leakage of the toner from occurring before the predetermined number of images are formed. Where the entire outer peripheral surface of the roller body is coated with the resin coating layer, the friction coefficient μ of the resin coating layer cannot be set at less than 0.25. This is because the developing performance of the developing roller is deteriorated.